In general, the top face of a piston of a direct fuel injection diesel engine is formed as a flat face, but a direct fuel injection diesel engine for which the top face of a piston projects in a pentroof shape is known from Patent Publication 1 below.
With regard to this direct fuel injection diesel engine, since the height of a peripheral wall portion of a cavity recessed in the top face of the pentroof-shaped piston changes in the circumferential direction, a vertical injection direction of fuel from a fuel injector is set according to the height of the peripheral wall portion, thus preventing flame from escaping from the cavity. Specifically, when the peripheral wall portion is low the direction of injection is set low, and when the peripheral wall portion is high the direction of injection is set high.
The peripheral wall portion of the cavity is a cylindrical face parallel to the piston central axis, fuel injected from the fuel injector at different angles of injection collides with the peripheral wall portion of the cavity on the cylindrical face, and the collision angle at which the injected fuel collides with the peripheral wall portion of the cavity (angle formed on opening side of cavity by fuel injection axis and tangent to cavity at collision point) is determined only by the angle of injection of fuel.
Patent Publication 1: Japanese Utility Model Registration Application Laid-open No. 62-52232